Field of the Invention
The present invention relates to a predistribution tray with scale collection functionality. The predistribution tray is typically located above a fine distribution tray in a vessel where a vapor phase and a liquid phase are flowing concurrently downward. The main purpose of the tray is to provide predistribution of liquid to the fine distribution tray, to remove larger scales and other larger solid impurities from the process stream, and to reduce any high velocities of the process stream before the stream reaches the fine distribution tray. The predistribution tray is suited for, but not limited to, the application of predistribution of hot hydrogen-rich treatgas and hot hydrocarbon liquid at the inlet of trickle bed reactors or hydroprocessing reactors, such as hydrotreating or hydrocracking reactors.
Description of Related Art
Several approaches and devices have been proposed for scale collection or filtering and for predistribution of liquid to a fine liquid distribution tray in two-phase concurrent downflow vessels, such as trickle bed catalytic reactors, in order to avoid solid contaminants in the feed stream collecting in the catalyst bed or on the fine distribution tray, causing excessive pressure drop and/or reduced apparent catalyst activity, and in order to protect the fine distribution tray from high velocity streams. The majority of these approaches or devices belong to one of the five groups mentioned below:
Group 1: Fluid by-Pass of Fouled Bed
An example of this approach is given in U.S. Pat. No. 4,380,529. An upper catalyst bed 22 is provided with by-pass tubes 23 and 24. When the upper catalyst bed is clean and the pressure drop across this bed therefore is low, only small amounts of vapor and liquid is claimed to bypass through the tubes 23 and 24. When the upper catalyst bed gradually becomes fouled and the pressure drop across the bed is increased, then larger amounts of vapor and liquid will by-pass the bed. As a result, the overall reactor pressure drop is reduced, and the process unit can stay in operation for a longer period, before the reactor pressure drop exceeds the available pressure from pumps and compressors in the process unit. The approach of fluid by-pass of the fouled bed has the disadvantage that the active catalyst is by-passed, so that the conversion of reactants to products is reduced. Also, in hydroprocessing reactors, if hydrogen is by-passed around a catalyst bed, then the rate of coke formation in that catalyst bed is increased. Coke formation results in high rates of catalyst deactivation and increased bed pressure drop.
Another example of fluid by-pass of a fouled catalyst bed is given in U.S. Pat. No. 6,692,705, where fluid is bypassed through a bypass tube 1 into a cage 2 with perforations 9 into a lower portion of the catalyst bed 5.
Group 2: Baskets Immersed in the Catalyst Bed
This was one of the first approaches used in hydroprocessing reactors to prevent plugging of the catalyst bed inlet with larger scales and solid contaminants. An example of baskets immersed in the catalyst bed is given in U.S. Pat. No. 3,112,256. Baskets 30 are immersed down into the inert topping layer, such as ceramic balls 32, and down into the active main catalyst bed 34. The upper edges of the baskets 30 are normally flush with the top of the inert topping layer 32. The baskets 30 increase the flow area available for fluid flow into the bed and thus lower the pressure drop for fluid entry into the bed. Therefore, as the inlet to the bed becomes fouled, the increase in bed pressure drop is lower for a bed with baskets 30 than for a bed without baskets 30.
The significant drawback of using baskets at the bed inlet is that the baskets 30 significantly deteriorate the fluid distribution provided by the fine distribution tray 18. In addition, reactants are bypassed, through the baskets, across the upper layer of catalyst. As a result the apparent catalyst activity is reduced when baskets at the inlet of the catalyst bed are used to reduce the bed pressure drop.
Group 3: Graded Guard Beds
Today, probably the most widely used industrial method to protect a fixed catalyst bed from solid impurities is by using graded guard beds of inert or catalyst particles at the reactor inlet. Typically, particle size, shape, and catalytic activity are graded, so that the particle size and the void fraction are gradually reduced, and the catalytic activity of the particles is gradually increased in the downward fluid flow direction in the reactor.
An example of a graded guard bed is given in U.S. Pat. No. 4,615,796. The reactor 1 has graded layers of particles 2, 3, 4, 5, and 6 in order to protect the main catalyst bed from solid contaminants. The upper layers are large particles with wide flow channels for fluid flow between the particles, and the lower layers are small particles with narrow flow channels for fluid flow between the particles. By having these graded layers, the solid contaminants will travel further down into the bed before they are trapped by the narrow flow channels. Also, the upper layers typically have high void fractions. For these two reasons the total volume available for deposit of solid contaminants in the space between the particles is increased, and consequently the rate of reactor pressure drop increase is lower when graded guard beds are used.
The disadvantage of using graded guard beds for accumulation of the solid contaminants in the feed is that the guard beds take up a significantly height of the straight part of the reactor 1. The graded layers of particles used to protect the main catalyst bed have low or no catalytic activity, and consequently the conversion of reactants to products in the reactor 1 is reduced.
The approaches and devices of group 1, 2, and 3 are located downstream from the fine distribution tray and thus do not provide predistribution of liquid to the fine distribution tray, nor do they provide protection of the fine distribution tray against fouling or high velocity streams.
Group 4: Filtering Trays Without Vapor By-Pass
An example of a filtering tray without vapor by-pass is given in U.S. Pat. No. 3,958,952. The entire process stream is forced to flow through filter units 4. The filtering tray without vapor by-pass removes the solid contaminants and therefore protects the fixed catalyst bed from fouling, so that the increase in pressure drop across the catalyst bed is reduced. Instead, the increase in pressure drop occurs across the filtering tray itself, resulting in increased reactor pressure drop and, at some point in time, shut down of the reactor is required for filter unit replacement or cleaning. See line 9-15 column 4. Shutdown of the reactor, and personnel entry into the reactor of a hydroprocessing unit is normally only done during catalyst replacement, since this operation is time-consuming and expensive. Another disadvantage of the design is that the filtering tray does not provide proper predistribution of the liquid to the fine distribution tray 3. Therefore, liquid level gradients may develop on the fine liquid distribution tray 3 as liquid is flowing from one area of this tray to another. These liquid level gradients will reduce the distribution performance of the fine distribution tray.
Another example of a filtering tray without vapor by-pass is given in U.S. Pat. No. 4,239,614. This filtering tray has annular beds of particles 4, 6, and 7. The entire process stream is forced to flow through these beds of particles, and solid contaminants will accumulate upstream from and in the particle beds. The tray has the same disadvantages as mentioned for U.S. Pat. No. 3,958,952.
Group 5: Filtering Trays with Vapor By-Pass
The benefit of all the filtering trays with vapor by-pass is that the process stream can flow through the tray even when the filter is plugged or full. The pressure drop across the tray is low even when the filters are full.
A first example of a filtering tray with vapor by-pass is given in U.S. Pat. No. 3,824,081. The filtering tray 5 is provided with a vapor opening at the tray center. A weir 7 surrounds this vapor opening and thus forms a vapor chimney. The tray 5 is provided with wire mesh baskets 6. During operation, the vapor flows through the vapor chimney, the liquid collects on tray 5, behind weir 7, and flows into the baskets 6, and through wire mesh or screen 47. Scales and solid contaminants are thus collected in the baskets 6. The drawback of the specific design is that the filtering tray provides poor predistribution of liquid to the fine distribution tray 40. Therefore, liquid level gradients may develop on the fine liquid distribution tray 40, as liquid is flowing from one area of the tray to another. These liquid level gradients will reduce the distribution performance of the fine distribution tray 40. Another disadvantage is that the height of the filtering tray has to be large in order to provide the required basket volume for collection of the scales and particles. To accommodate the filtering tray, the height of the reactor will have to be increased, which is associated with large additional costs.
A second example of a filtering tray with vapor by-pass is given in U.S. Pat. No. 8,487,151. The filtering tray consists of a perforated tray 1 with a filtration bed comprising different layers of particles I, II, III, and IV (FIG. 1). Vapor chimneys 3 are routed through the particle layers and the perforated tray 1. During operation, the vapor passes through the chimneys 3, while the liquid is trickling down through the filtration bed and through the perforations 7 in the tray 1. Larger solid impurities will accumulate in the void space between the particles of the filtration bed. At some point in time, the liquid may no longer be able to pass through the filtration bed, and the liquid will overflow the central tube 4 to the fine distribution tray 10. Again the drawback of this design is that the filtering tray provides poor predistribution of liquid to the fine distribution tray 40. This is especially true when the filtering bed in some areas gets plugged by solid impurities, and the liquid flow through these areas stops. When the filtering tray is full and the liquid therefore passes through overflow pipe 4, all the liquid feed may enter the fine distribution tray 10 near the centerline of the reactor. This situation is known to result in large liquid level gradients on the fine liquid distribution tray 10, because the radial outward liquid mass flux near the reactor centerline gets very large. The liquid level gradients will reduce the distribution performance of the fine distribution tray 10. Another disadvantage is that the height of the filtering tray has to be large, in order to provide the required volume of the void space between the filtering particles for deposit of the scales and solid contaminants. The height of the reactor will have to be increased to accommodate the filtering tray, which is associated with large additional costs.
A third example of a filtering tray with vapor bypass is given in U.S. Pat. No. 8,329,974 and US patent application US 2013/0064727 A1. The filtering tray consists of a tray with perforations 12. A granular filtration bed comprising three different layers rests on the perforated tray. The tray is provided with chimneys 3 having vapor openings 6, and liquid slots 4, and is surrounded by cylindrical screens 8. During operation, the vapor by-passes the filtration bed through the vapor openings 6 and the chimneys 3 to the active catalyst bed 10. In the start of the cycle, when the filtration bed is clean, the liquid is passing through the filtration bed and through the perforations 12 to the active catalyst bed 10. As the filtration bed gets plugged, the liquid flow stops in the plugged areas, and liquid will instead pass through the liquid slots 4 and chimneys 3 to the active catalyst bed 10. The drawback of this filtering tray is that as some areas of the filtering bed become plugged, the liquid flow through these areas stops, and the active catalyst located below the plugged areas of the filtering tray receives no liquid feed.
Chimney trays are widely used to distribute liquid evenly to catalyst beds, but uniform liquid distribution from a chimney tray requires that all chimneys are exposed to approximately the same liquid level. With the filtering tray as disclosed in the above example, all chimneys will not be exposed to the same liquid level, because of the flow resistance of the filtering bed, and because some areas of the filtering bed will become more fouled than other areas and thus further increase the flow resistance of the bed. Due to the large flow resistance of the filtering bed, chimneys 3 located in an area receiving large liquid amounts from above will pass large quantities of liquid to bed 10, and chimneys 3 located in an area receiving small liquid amounts from above will pass small quantities of liquid to bed 10. The consequences of the non-uniform liquid feed distribution to the active catalyst bed 10 are lower overall conversion of reactants to products, and radial temperature differences in the active catalyst bed 10. Another disadvantage of the filtering tray is that the tray will have to be located in the straight portion of the reactor as, shown in FIG. 1 of U.S. Pat. No. 8,329,974, and that the height of the filtering tray has to be large in order to provide the required volume of the void space between the filtering particles for deposit of scales and solid contaminants. Any additional height of a hydroprocessing reactor is associated with large extra costs.